a) Field of the Invention
The present invention relates to a CFRP (carbon fiber-reinforced plastic)-made optical cylinder used typically in astronomical telescopes, etc.
b) Description of the Related Art
Cylindrical articles used in industries and various devices or apparatuses are produced generally using various metallic materials or organic materials (e.g. resins). The requirements for these cylindrical articles depend upon their applications, and are extremely high in some cases. With respect to the optical cylinder intended by the present invention, no fully satisfactory material has been found because of the requirements.
In optical cylinders, for example, cylinders of astronomical telescopes, since the astronomical observation often lasts as long as several hours even when an ordinary type telescope is used, the deviation of an image from the focal length set in advance, due to the change in atmospheric temperature during the observation is as large as about 0.1 mm in the case of an iron-made cylinder of 1 m in length when the atmospheric temperature changes by 10.degree.C. Thus, when the telescope cylinder is made of iron having a thermal expansion coefficient .alpha. of 10.times.10.sup.-6 /.degree.C., the deviation is as mentioned above, and when the telescope cylinder is made of aluminum having an .alpha. of 24.times.10.sup.-6 /.degree.C., the deviation is as large as 0.24 mm.
The change in atmospheric temperature during the observation gives rise to air layers of different specific gravities in the cylinder when the cylinder is made of a metal material of high thermal conductivity, which produces a convection therein and consequently gives a distorted image. Incidentally, the thermal conductivity of iron is 50 w/mk and that of aluminum is 237 w/mk. The formation of air layers of different specific gravities is influenced not only by the thermal conductivity of the cylinder material but also by its specific heat. Incidentally, the specific heat of iron is 3.6 J/cm.sup.3..degree.C.
In the cylinders made of materials having the above-mentioned levels of the thermal expansion coefficient, thermal conductivity, specific heat, etc., the frequent correction of the optical axis is required, making it difficult to maintain a sharp image during the observation time.
Also in the cylinders made of conventional materials, since the specific modulus of each material is small, the cylinder weight is large in order to have desired rigidity, making the balance weight large and requiring a tripod, etc. made of a material of high rigidity. Incidentally, the specific modulus which is a quotient obtained by dividing modulus by specific gravity is 2.7.times.10.sup.9 mm.sup.2 in the case of iron and 5.4.times.10.sup.9 mm.sup.2 in the case of aluminum.